30 Smyth^ Jr. — Peirography of Dikes in Syracuse^ N. Y. 



Manheim rock and, moreover, seemed fairly fresh. This was 

 searched, with greatest care, for melilite, but to no purpose. 

 In the present study, three sections of the rock have been 

 examined, one from an old specimen and two from new material. 

 In the first, a few very small individuals of melilite were found, 

 so inconspicuous that their discovery can be regarded as little 

 more than accidental. But in the other two sections, the 

 melilite is present in some quantity. In habit and general 

 appearance it is very like the Manheim melilite, though less 

 abundant. Like the latter, it often shows abnormally high 

 double refraction, possibly the result of alteration, and, as a 

 rule, is optically positive, although the negative variety is also 

 present. 



From the foregoing, it is evident that in rocks as thoroughly 

 serpentinized as are those of the Syracuse dikes, melilite may be 

 nearly or quite obliterated ; and it is extremely probable that 

 perfectly fresh specimens would show melilite in all of these 

 dikes. Perhaps an exception should be made in the case of 

 the DeWitt dike, but even here there would seem to be suffi- 

 cient alteration to mask the melilite if originally present in 

 small quantities. Apparently, then, the petrographic affinities 

 of this group of dikes is with the melilite rocks rather than 

 with the peridotites, and this is interesting in bringing them 

 into close relationship with the Manheim dikes, and in deter- 

 mining another occurrence of a rare variety of rock. Of 

 course, incomparably the most important contribution to our 

 knowledge of the Syracuse rocks was the establishment by 

 Williams'^ of their igneous nature, but all new data in regard 

 to the locality are worthy of record. 



In a note appended to his description of the De Witt dike, 

 Prof. Kempt gives a brief resume of the occurrences of igneous 

 rocks in Central I^ew York, including two instances of bowlders. 

 Another case of the latter kind may be mentioned here. 



In the Hamilton Collesre collection are two specimens 

 labelled " Hornblende Boufder, Syracuse, IST. Y." The rock 

 is coarsely porphyritic, with very little groundmass, and so 

 peculiar in aspect that the writer had a section made from it 

 some years since. Under the microscope this shows idio- 

 morphic pj^roxene of pale violet color, with a somewhat smaller 

 amount of olivine. The individuals of both range up to 

 10-12""^ in diameter, and the two minerals make up perhaps 

 90 per cent of the rock, being held together by a scanty ground- 

 mass of minute laths of plagioclase. While it is quite possible 

 that the rock is of Canadian origin, still it may well belong 

 with the other bowlders referred to, in a group of basic erup- 

 tives breaking through the New York Paleozoic rocks. 



Hamilton College, Clinton, N. Y. 



*0p. cit. and Bull. Geol. Soc. Am., i, pp. 533-534. 

 f Op. cit., p. 462. 



